1. Field of the Invention
The present invention relates to a power circuit and a method for controlling the power circuit being capable of making a peak value of a source current adapted to supply source power to a load in which its power consumption changes periodically and of allowing an amount of a current drain to exceed an upper limit value within a set period of time.
The present application claims priority of Japanese Patent Application No. 2002-045219 filed on Feb. 21, 2002, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, as terminals of portable devices become multifunctional to achieve connections or access to the Internet, distribution of moving pictures, or a like, both a transmission speed and an amount of data communications tend to increase. Moreover, it is essential that these terminals of portable devices be made not only multifunctional but also smaller and more lightweight from a viewpoint of convenience for use.
On the other hand, as terminals of portable devices become multifunctional, an amount of power being consumed tends to increase and therefore it is required that a cell being used as a power source has a large capacity. However, such a large capacity of a cell serving as a power source leads to a larger scale of and a larger weight of terminals of portable devices, which become contradictory to a demand for being made smaller and lightweight being required for terminals of portable devices to improve performance.
To satisfy such contradictory demands in terminals of portable devices, a cell having higher energy density or having lower internal impedance is being developed, however, such presently available contrivance does not achieve a satisfactory result. Moreover, development of technology to enhance discharge efficiency of a cell is being subject to review. In the case of terminals of portable devices, in many cases, performance of a cell is evaluated by using operating time of portable devices as a specified value. Here, the operating time refers to elapsed time during which portable devices can be operated normally and is defined as time being elapsed before a voltage of the cell reaches a lowest value of a voltage that can drive portable devices.
To improve discharge efficiency of a cell, matching between a lowest voltage that can drive the above portable devices and a voltage that terminates discharge of the cell is effective and therefore methods which enable portable devices to be operated at a low voltage by using a DC—DC (direct current to direct current) converter circuit or the like as a power source and which enable calibration of a discharge terminating voltage of a cell by optimizing a number of serial cells are conventionally employed.
However, in the case of such portable devices in which their loads vary with time which is specific to digital devices prevailing in recent years as devices operating according to a TDMA (Time Division Multiple Access) method, digital data transfer systems, or likes, since their cell voltage changes over time, matching between the lowest driving voltage and the discharge terminating voltage becomes difficult and therefore a conventional power source circuit has a problem in that it cannot operate a cell fully effectively.
Moreover, in the case of devices being supplied with source power from a personal computer, or the like, serving as a host according to a USB (Universal Serial Bus) with specifications for the Internet, there is a problem in that, since an upper limit is imposed on a current that can be employed in the USB, devices that consume a current exceeding the upper limit can not be used.
Thus, a conventional power circuit presents a problem in that effective use of a cell serving as a power source is impossible and devices consuming a current exceeding such a limit value cannot be used.